Fiber optic amplifier

ABSTRACT

An amplifier for use with fiber optic systems comprises a neodymium YAG crystal placed in series with a signal-carrying optical fiber. The ND:YAG crystal is supplied by the optical fiber with both the signal to be amplified, and pumping illumination. The pumping illumination is coupled onto the optical fiber by a multiplexing coupler which is used to combine the signal to be amplified and illumination from a pumping illumination source onto a single optical fiber. The pumping illumination inverts the neodymium ions within the ND:YAG crystal. The signal to be amplified propagates through this crystal to stimulate emission of coherent light from the neodymium ions, resulting in amplification of the signal. Because this arrangement permits the ND:YAG crystal to be end-pumped with pumping illumination, and because the length of the ND:YAG crystal may be substantially greater than the absorption length for the crystal at the wavelength of the pumping illumination, virtually all of the pumping illumination may be absorbed within the ND:YAG crystal and used for amplification of the signal carried by the optical fiber.

BACKGROUND OF THE INVENTION

The concept of optical amplifiers, based upon the capability of certainmaterials, particularly on a macroscopic level, is well known. Thus, forexample, it is known to place a pumping light source and a singlecrystal neodymium-yttrium aluminum garnet (ND:YAG) rod, severalmillimeters in diameter and several centimeters in length, in a tubularreflective cavity. For example, the licht source and ND:YAG rod may belocated, respectively, to extend along the two foci of a cavity havingan elliptical cross section. In such an arrangement, light emitted bythe light source and reflected from the cavity walls will impinge uponthe ND:YAG rod. The light source is preferably selected to emitwavelengths corresponding to the absorption spectra of the ND:YAGcrystal so that the neodymium ions of the crystal are inverted to anenergy level above the upper lasing level. After inversion, an initialrelaxation of the neodymium ions through phonon radiation yields an ionpopulation at the upper lasing level. From this level, the ions willrelax to a lower lasing level, emitting light of a wavelength which ischaracteristic of the ND:YAG material. Advantageously, the lower lasinglevel is above the ground level for the ions so that a rapid,phonon-emitting relaxation will occur between the lower lasing level andthe ground level, enabling a high inversion ratio to exist between theupper and lower lasing levels within the pumped ions.

With the population so inverted, as is well known from laser technology,the ND:YAG will provide a very slow fluorescence, that is, randomemission of incoherent light. This spontaneous radiation, however, has aminimal effect on the amplifying rod, since the average lifetime of ionsin the inverted state is 230 microseconds, in ND:YAG.

If, after some of the neodymium ions of the ND:YAG rod have beeninverted, a light signal at the lasing frequency is transmitted throughthe rod, the light signal will trigger the relaxation of the neodymiumions, causing coherent emission of stimulated radiation, which willeffectively add to the transmitted signal, thus amplifying this signal.

The absorption length of the pumping illumination within the ND:YAGcrystal (i.e., the length of material through which the illuminationmust traverse before about 65% of the illumination is absorbed) istypically in the range between 2 and 3 millimeters, and thus the ND:YAGcrystals used in transverse pumping structures such as describedpreviously have had diameters at least this large so that the crystalcould absorb a substantial portion of the pumping radiation during theinitial reflection from the cavity walls and passage through thecrystal. If, during this initial traverse through the crystal, thepumping illumination is not absorbed, it is likely to be reflected bythe cavity walls back to the light source, where it will be reabsorbed,generating heat in the light source and reducing the overall efficiencyof the amplifier.

When such amplifiers are used in fiber optic systems, it has beenthought necessary, because of the large difference in diameter betweenthe optical fiber and the ND:YAG crystal, to use optical components,such as lenses, to focus light from the optical fiber into the ND:YAGrod, and the amplified light signal from the ND:YAG rod back intoanother fiber. Such optical systems require careful alignment and aresusceptible to environmental changes, such as vibration, and thermaleffects. Additionally, the optical components and the size of the ND:YAGrod make the amplifying system relatively large, and thus impracticalfor certain applications. Furthermore, the relatively large size of theND:YAG rod introduces beam wander within the rod due to thermal effects.Thus, the signal from the input fiber optic element will traversedifferent paths through the rod, a characteristic which is temperaturerelated and varies with time, so that the output light may be lost dueto the fact that the fiber will accept only light within a smallacceptance angle. Thus, as the beam within the ND:YAG rod wanders, theoutput signal may vary in an uncontrollable manner. Furthermore, thelarge size of the ND:YAG rod requires a large amount of input energy inorder to maintain a high energy density within the rod. Such large pumppower requires high output light sources, generating substantial heatwhich must be dissipated, typically by liquid cooling of the cavity.

While amplifiers of this type are useful in many applications, such assome communications applications, a use which puts severe limitationsupon the amplification system is a recirculating fiber optic gyroscope.With such gyroscopes, an optical fiber, typically a kilometer or more inlength, is wound into a loop, and a light signal is recirculated withinthe loop in both directions. Motion of the loop causes a phasedifference between the counter-propagating light signals which may beused to measure gyroscope rotation. It is advantageous, because thephase shift induced in one rotation is relatively small and becauseperiodic outputs relating to rotation are required, to recirculate inputlight within the loop as many times as possible.

In traversing a kilometer of optical fiber, an optical signal willtypically lose 30 to 50 percent of its intensity. An amplifier, ifcapable of amplifying the bidirectional counter-propagating lightsignals, would permit a light signal to propagate many more times withinthe loop, if the amplifier were placed in a series with the loop, andprovided a gain equal to the propagation loss.

Unfortunately, the relatively large size, high power requirements causedby relatively inefficient performance, beam wander effects, and coolingrequirements of prior art ND:YAG rod amplifiers makes such amplifiersrelatively impractical for high accuracy gyroscopes. These factors, ofcourse, also limit the utility of such amplifiers in other applications,such as communication networks.

SUMMARY OF THE INVENTION

The disadvantages associated with crystal rod amplifiers are alleviatedin the present invention. This invention permits end pumping of theND:YAG material, and thus, completely avoids the requirement for a largediameter for this crystal which is inherent in side pumpingarrangements. The ND:YAG fiber may thus be made extremely small indiameter in comparison with prior art rod amplifiers, since the pumpingillumination is absorbed along the length of the fiber, rather thanacross its width. This results in a higher concentration of pumpingillumination within the small diameter of the ND:YAG crystal and thus ahigher potential gain for the amplifying structure.

In order to accomplish this end pumping, the ND:YAG material is formedas a small diameter fiber and is placed in series with the optical fiberwhich is transmitting the signal to be amplified.

Adjacent to the ND:YAG fiber, the optical fiber passes through amultiplexing optical coupler. Within this multiplexing coupler, a pairof optical fibers are arranged with a carefully selected interactionlength to provide a high fiber-to-fiber coupling efficiency at thewavelength of the pumping source, but a low coupling efficiency at thewavelength of the signal to be amplified. This results in a coupling ofthe pumping illumination into the optical fiber which is carrying thesignal, and thus into the ND:YAG fiber, while substantially eliminatingloss to the optical signal which is to be amplified, since this signalis not coupled from the optical fiber of the multiplexing coupler.

Because the present invention permits the pumping illumination to becoupled into the optical signal fiber for guiding to the end of theND:YAG fiber, the diameter of the ND:YAG fiber need not exceed theabsorption length at the pumping wavelength, since the pumpingillumination is effectively absorbed in a direction along the axis ofthe ND:YAG fiber rather than perpendicular to that axis.

Using this arrangement, pumping illumination can be continuouslysupplied to the amplifying ND:YAG fiber without interfering with itssignal carrying characteristics. Thus, since a four-port coupler is usedfor multiplexing the pumping illumination to the amplifying fiber, theends of the amplifying fiber are always available for direct signalcoupling to the optical fibers within the optical system so that carefultime sequencing between the application of pumping illumination and thesignal to be amplified is not necessary.

In order to achieve uniform bidirectional amplification within theND:YAG crystal, pumping illumination may be supplied by multiplexingcouplers arranged at both ends of the ND:YAG fiber, providing asymmetrical inversion population along the length of this fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention are best understoodthrough reference to the drawings, in which:

FIG. 1 is a cross sectional view of the fiber optic coupler used as amultiplexer in the present invention showing a pair of fiber opticstrands mounted in respective arcuate grooves with a given radius ofcurvature of respective bases;

FIGS. 2 and 3 are cross sectional views of the coupler of FIG. 1 takenalong lines 2--2 and 3--3, respectively;

FIG. 4 is a perspective view of the lower base of the coupler of FIG. 1separated from the other base to show its associated fiber mounting andthe oval-shaped facing surface of the fiber;

FIG. 5 is a schematic diagram showing the evanescent fields of the pairof fibers overlapping at the interaction region;

FIG. 6 is a chart showing relative coupled power versus signalwavelength for a fiber coupler having a minimum fiber spacing of 4microns, an offset of 0 micron, and a fiber radius of curvature of 25centimeters;

FIG. 7 is a chart of relative coupled power versus signal wavelengthssimilar to FIG. 6 but with a fiber radius of curvature of 200centimeters;

FIG. 8 is a chart of relative coupled power versus signal wavelength fora fiber coupler having a minimum fiber spacing of 4 microns, a fiberradius of 200 centimeters, and a selectable fiber offset;

FIG. 9 is a schematic diagram showing the directional amplifier of thepresent invention;

FIG. 10 is a diagram showing the absorption spectrum of ND:YAG at 300°K.;

FIGS. 11a and 11b are simplified energy level diagrams of a four-levellaser using a doped material, such as ND:YAG;

FIG. 12 is a schematic diagram of a symmetrical bidirectional amplifierin accordance with the present invention;

FIG. 13 is a schematic diagram of an amplifier system, including amultiplexing coupler for signal insertion and signal sensing in a closedfiber loop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to gain a detailed understanding of the operation of the fiberoptic amplifier of this invention, it is necessary to understand themanner in which a fiber optic coupler can be constructed to selectivelycouple a first optical frequency while not coupling a second opticalfrequency. The apparatus, as well as methods for constructing theapparatus, required for such selective coupling, are disclosed incopending patent application entitled "Passive Fiber Optic Multiplexer",filed in the United States Patent Office Nov. 9, 1981, bearing Ser. No.319,301, now U.S. Pat. No. 4,556,279, and listing Herbert J. Shaw andMichael J. F. Digonnet as inventors. That application is assigned to theassignee of the present invention. That application is herebyincorporated herein by reference. Nonetheless, the principalcharacteristics of that device and its method of manufacture aredescribed below.

This invention utilizes a passive multiplexer which utilizes a fiberoptic coupler. This coupler 10 is illustrated in FIGS. 1-4, and includestwo strands 12A and 12B of a single mode fiber optic material. The fiberstrands 12A and 12B, in the embodiment shown, have a core radius of 2microns, a core refractive index of 1.46 and a cladding index of 1.456.The strands 12A and 12B are mounted in longitudinal arcuate grooves 13Aand 13B, respectively, formed in optically flat confronting surfaces 14Aand 14B, respectively, of rectangular bases or blocks 16A and 16B,respectively. The block 16A with the strand 12A mounted in the groove13A will be referred to as the coupler half 10A and the block 16B withthe strand 12B mounted in the groove 13B will be referred to as thecoupler half 10B.

Each of the strands 12A and 12B comprise an optical fiber which is dopedto have a central core and an outer cladding. The strands 12A and 12Bmay comprise a commercially available fiber of quartz glass which isdoped to have a central core and an outer cladding. The index ofrefraction of the fibers 12A and 12B should be as nearly as possibleidentical, and both of the strands 12A and 12B should include a centralcore which is sufficiently small to provide single mode transmission atthe optical frequencies to be used. Thus, these strands 12A and 12Btypically have a core diameter on the order of 10 microns or less and acladding diameter on the order of 125 microns. In the embodimentdisclosed, the diameter of the strands 12 and their respective cores areexaggerated. As will be understood in more detail from the descriptionwhich follows, the fiber 12B is used to transmit the signal to beamplified while the fiber 12A is used to couple pumping illumination tothe fiber 12B. For this reason, the fiber 12B will be referred to as thesignal fiber while the fiber 12A will be referred to as the pumpingfiber.

The arcuate grooves 13A and 13B have a radius of curvature which is verylarge compared to the diameter of the fibers 12, and have a widthslightly larger than the fiber diameter to permit the fibers 12, whenmounted therein, to conform to a path defined by the bottom walls of thegrooves 13. The depth of the grooves 13A and 13B varies from a minimumat the center of the blocks 16A and 16B, respectively, to a maximum atthe edges of the blocks 16A and 16B, respectively. This advantageouslypermits the fiber optic strands 12A and 12B, when mounted in the grooves13A and 13B, respectively, to gradually converge toward the center anddiverge toward the edges of the blocks 16A, 16B, thereby eliminating anysharp bends or abrupt changes in direction of the fibers 12 which maycause power loss through mode perturbation. In the embodiment shown, thegrooves 13 are illustrated as being rectangular in cross section,however, it will be understood that other suitable cross sectionalcontours which will accommodate the fibers 12 may be used alternatively,such as a U-shaped cross section or a V-shaped cross section.

At the centers of the blocks 16, in the embodiment shown, the depth ofthe grooves 13, which mount the strands 12, is less than the diameter ofthe strands 12, while at the edges of the blocks 16, the depth of thegrooves 13 is preferably at least as great as the diameter of thestrands 12. Fiber optic material was removed from each of the strands12A and 12B e.g. by polishing, to form the respective oval-shaped planarsurfaces 18A, 18B, which are coplanar with the confronting surfaces 14A,14B, respectively. These surfaces 18A, 18B, will be referred to hereinas the fiber "facing surfaces". Thus, the amount of fiber optic materialremoved increases gradually from zero towards the edges of the block 16to a maximum towards the center of the block 16. This tapered removal ofthe fiber optic material enables the fiber cores to converge and divergegradually, which is advantageous for avoiding backward reflection andexcess loss of light energy.

In the embodiment shown, the coupler halves 10A and 10B are identical,and are assembled by placing the confronting surfaces 14A and 14B of theblocks 16A and 16B together, so that the facing surfaces 18A and 18B ofthe strands 12A and 12B are in facing relationship.

An index matching substance (not shown), such as index matching oil, isprovided between the confronting surfaces 14. This substance has arefractive index approximately equal to the refractive index of thecladding, and also functions to prevent the optically flat surfaces 14from becoming permanently locked together. The oil is introduced betweenthe blocks 16 by capillary action.

An interaction region 32 is formed at the junction of the strands 12, inwhich light is transferred between the strands by evanescent fieldcoupling. It has been found that, to insure proper evanescent fieldcoupling, the amount of material removed from the fibers 12 must becarefully controlled so that the spacing between the core portions ofthe strands 12 is within a predetermined "critical zone". The evanescentfields extend into the cladding and decrease rapidly with distanceoutside their respective cores. Thus, sufficient material should beremoved to permit each core to be positioned substantially within theevanescent field of the other. If too little material is removed, thecores will not be sufficiently close to permit the evanescent fields tocause the desired interaction of the guided modes, and thus,insufficient coupling will result. Conversely, if too much material isremoved, the propagation characteristics of the fibers will be altered,resulting in loss of light energy due to mode perturbation. However,when the spacing between the cores of the strands 12 is within thecritical zone, each strand receives a significant portion of theevanescent field energy from the other strand, and good coupling isachieved without significant energy loss. The critical zone isillustrated schematically in FIG. 5 as including that area, designatedby the reference numeral 33, in which the evanescent fields, designatedby reference numerals 34A and 34B, of the fibers 12A and 12B,respectively, overlap with sufficient strength to provide coupling,i.e., each core is within the evanescent field of the other.

The blocks or bases 12 may be fabricated of any suitable rigid material.In one presently preferred embodiment, the bases 12 comprise generallyrectangular blocks of fused quartz glass approximately one inch long,one inch wide, and 0.4 inch thick. In this embodiment, the fiber opticstran 12 are secured in the slots 13 by suitable cement 38, such asepoxy glue. One advantage of the fused quartz blocks 16 is that theyhave coefficient of thermal expansion similar to that of glass fibers,and this advantage is particularly important if the blocks 16 and fibers12 are subjected to any heat treatment during the manufacturing process.Another advantage of the fused quartz blocks is that, as they are madeof the same material as the optical fiber, they polish at the same rateas the optical fiber, and thus, provide a continuous support to thefiber during polishing. Other suitable materials for the block 16include silicon, which also has excellent thermal properties for thisapplication.

OPERATION OF THE COUPLER 10

The coupler 10 includes four ports, labeled A, B, C, an D in FIG. 1.When viewed from the perspective of FIG. 1, ports A and C, whichcorrespond to strands 12A and 12B, respectively, are on the left-handside of the coupler 10, while the ports B and D, which correspond to thestrands 12A and 12B, respectively, are on the right-hand side of thecoupler 10. For the purposes of discussion, it will be assumed thatinput light is applied to port A. This light passes through the couplerand is output at port B and/or port D, depending upon the amount ofpower that is coupled between the strands 12. In this regard, the term"normalized coupled power" is defined as the ratio of the coupled powerto the total output power. In the above example, the normalized coupledpower would be equal to the ratio of the power at port D to the sum ofthe power output at ports B and D. This ratio is also referred to as the"coupling efficiency", and when so used is typically expressed as apercent. Thus, when the term "normalized coupled power" is used herein,it should be understood that the corresponding coupling efficiency isequal to the normalized coupled power times 100. In this regard, testshave shown that the coupler 10 has a coupling efficiency of up to 100%.However, it will also be seen that the coupler 10 may be "tuned" toadjust the coupling efficiency to any desired value between zero and themaximum. Such tuning may be accomplished e.g. by relatively laterallysliding the fibers in a direction perpendicular to their length. Therelative positions of the fibers may be defined in terms of theiroffset, i.e. the distance between the central axes of the fiber cores,measured in the direction perpendicular to their length, along thesurfaces 14. Thus, when the oval surfaces 18 are superimposed, theoffset is zero, and the offset increases as the fibers 12 are laterallyseparated by relatively sliding the blocks 16.

The coupler 10 is highly directional, with substantially all of thepower applied at one side of the coupler being delivered to the otherside of the coupler. The coupler directivity is defined as the ratio ofthe power at port D to the power at port C, with the input applied toport A. Tests have shown that the directionally coupled power (at portD) is greater than 60 db above the contra-directionally coupled power(at port C). Further, the coupler directivity is symmetrical. That is,the coupler operates with the same characteristics regardless of whichside of the coupler is the input side and which side is the output side.Moreover, the coupler 10 achieves these results with very low throughputlosses. The throughput loss is defined as the ratio of the total outputpower (ports B and D) to the input power (port A) subtracted from one(i.e., 1-(P_(B) +P_(D))/P_(A)). Experimental results show thatthroughput losses of 0.2 db have been obtained, although losses of 0.5db are more common. Moreover, these tests indicate that the coupler 10operates substantially independently of the polarization of the inputlight applied.

The coupler 10 operates on evanescent field coupling principles in whichguided modes of the strands 12 interact, through their evanescentfields, to cause light to be transferred between the strands 12. Aspreviously indicated, this transfer of light occurs at the interactionregion 32. The amount of light transferred is dependent upon theproximity and orientation of the cores, as well as the effective lengthof the interaction region 32. As will be described in detail below, theamount of light transferred is also dependent on the wavelength of thelight. The length of the interaction region 32 is, in turn, dependentupon the radius of curvature of the fibers 12, and, to a limited extent,the core spacing, although it has been found that the effective lengthof the interaction region 32 is substantially independent of corespacing. However, the "coupling length", i.e., the length within theinteraction region 32 which is required for a single, complete transferof a light signal from one fiber 12 to the other, is a function of corespacing, as well as wavelength. In one exemplary embodiment, employingan edge-to-edge core spacing of about 1.4 microns, and a radius ofcurvature on the order of 25 centimeters, the effective interactionregion is approximately one millimeter long at a light signal wavelengthof 633 nm. Because the coupling length at 633 nm is also one millimeterin such a coupler, the light makes only one transfer between the strands12 as it travels through the interaction region 32. However, if thelength of the interaction region 32 is increased, or core spacingdecreased, a phenomenon referred to herein as "overcoupling" will occur,since the coupling length is shorter than the effective interactionlength. Under these circumstances, the light will transfer back to thestrand from which it originated. As the interaction length is furtherincreased, and/or the core spacing further decreased, the effectiveinteraction length increases relative to the coupling length, and atleast some of the light transfers back to the other strand. Thus, thelight may make multiple transfers back and forth between the two strands12 as it travels through the region 32, the number of such transfersbeing dependent on the length of the interaction region 32, the lightwavelength (as described below), and the core spacing.

Since the coupling length in a single mode fiber coupler, as describedabove, shows a strong dependence on signal wavelength, as described indetail in the copending application incorporated above, it is possiblewith properly chosen geometrical parameters for the coupler 10, tototally couple one signal wavelength while a second signal wavelengthremains essentially uncoupled. This phenomenon permits the combinationof two signals fed into the ports on one side of the coupler 10. Thus,as shown in FIG. 1, if a pumping signal having a wavelength λ₁ is fedinto port A of coupler 10, and a signal to be amplified, having awavelength λ₂ is coupled to port C, and the geometry is properlyselected, both signals can be combined at port D, with virtually nolight output at port B.

To illustrate this wavelength dependence, FIG. 6 provides a plot ofcoupled power versus signal wavelength in the visible and near infraredspectrum for a particular coupler geometry. Because for this couplerconfiguration the effective interaction length of the coupler is an oddmultiple of the coupling length for the wavelength 720 nm, but an evenmultiple of the coupling length for the wavelength 550 nm, thewavelength 720 nm will be 100% coupled, while the wavelength 550 nm willbe effectively uncoupled. With different efficiencies, differentwavelengths may be combined or separated. For instance, 590 nm and 650nm may be separated or combined at an 80% efficiency.

Virtually any pair of wavelengths (λ₁,λ₂) may be efficiently combined orseparated so long as the effective interaction length is an evenmultiple of the coupling length for one wavelength and an odd multiplefor coupling length for the other wavelength. As the number of couplinglengths within the effective interaction length increases, theresolution of the multiplexer is enhanced. As is described in detail inthe incorporated reference, the multiplexer resolution may be enhancedby increasing the radius of curvature of the fibers 12A,12B. Providedthat the interaction length of the coupler is large enough, virtuallyany two signals may be exactly mixed or separated, regardless of howclosely spaced their wavelengths are.

The interaction length is a function of wavelength, and the resolutionis approximately proportional to (R)^(-1/2). As R increases, theeffective interaction length increases, and becomes a higher multiple ofthe coupling length, improving resolution. This result is illustrated inFIG. 7, which is comparable to the graph of FIG. 6, except that theradius of curvature has been increased to 200 centimeters. Asanticipated, this increase in radius improves the coupler resolutionnear g=600 nm from approximately 170 nm in 25 centimeter radius exampleof FIG. 6 to approximately 60 nm in the 200 centimeter case.

The resolution of a multiplexing coupler depends on two independentparameters, H (fiber spacing) and R (radius of curvature of the fibers).For a given pair of signal wavelengths, efficient mixing may be achievedby first properly selecting a fiber spacing H for the coupler whichyields a large wavelength dependence for the wavelengths of interest(choice of H), and then by selecting a radius of curvature which yieldsa resolution equal to the difference between the wavelengths (choice ofR).

After the resolution of the coupler has been set in accordance with thewavelengths to be separated, the coupler may be tuned to preciselyadjust the coupling lengths for the wavelengths of interest so that theeffective interaction length is an even multiple of the coupling lengthof one wavelength and an odd multiple of the coupling length of theother wavelength. This is accomplished by offsetting the fibers bysliding the blocks 16A,16B (FIG. 1) relative one another in a directionnormal to the axis of the fibers 12A,12B. Such an offset has the effectof increasing the minimum fiber spacing H and increasing the effectiveradius of curvature of the fibers. If the required offset is smallenough, it will not upset the multiplexer resolution. This stems fromthe fact that the separation H of a large radius coupler changes rapidlywith fiber offset in comparison to changes in the effective radius ofcurvature with fiber offset.

To illustrate this tunability of multiplexing couplers, FIG. 8 providesa plot of relative coupled power versus wavelength for three increasingvalues of fiber offset (0 microns, 0.5 microns, and 1.0 microns). Thecurve is seen to shift toward increasing wavelengths the offsetincreases, while the period of oscillation (or resolution) remainsvirtually unchanged. In this particular example (R=200 cm, H=4 microns),a 1-micron offset shifts the curve by approximately 45 nm.

OVERALL OPERATION OF THE AMPLIFIER

Referring now to FIG. 9, the manner in which the amplifier of thepresent invention utilizes the wavelength multiplexing properties of thecoupler 10 to provide pumping illumination to energize ND:YAG fiber willbe described.

A source of pumping illumination 42 is coupled to the fiber 12A of thecoupler 10 to provide pumping illumination at port A of the multiplexingcoupler 10, and a signal to be amplified is coupled to one end of thefiber 12B at port C of the coupler 10. The pumping illumination from thesource 42 and the signal to be amplified are combined at port D at thecoupler 10 through the multiplexing action of the coupler, as explainedabove. Thus, the coupler 10 is adjusted to have a 100% couplingefficiency at the wavelength of the source and a 0% coupling efficiencyat the wavelength of the signal input at port C. This pair of signalwavelengths is supplied to an ND:YAG crystal 44 which is coupled to thefiber 12b in the manner described below. The signal input at the port Cwill be amplified in the ND:YAG crystal 44 and the amplified signal willbe coupled from this crystal 44 to an optical fiber 46 for transmissionwithin the optical fiber system. The diameter of the ND:YAG crystal 44may be extremely small in comparison with the diameter of ND:YAG rodsused in prior art amplifiers. For example, an amplifier has beenconstructed in which the crystal 44 has a diameter of 100 microns. Evensmaller diameters are feasible, approaching the diameter of the singlemode fiber 12B. Coupling between the fiber 12B and the crystal 44 isenhanced as the diameter of the crystal 44 is reduced and signal gain isincreased since the density of pumping illumination from the source 42within the crystal 44 increases as the crystal 44 diameter is reduced.

ND:YAG AMPLIFICATION

Referring now to FIG. 10, which is a diagram of the absorption spectrumof the ND:YAG crystal 44 at 300° K., it can be seen that ND:YAG materialhas a relatively high optical density, and thus a short absorptionlength, at selected wavelengths. For this reason, it is advisable toselect the wavelength of the pumping illumination source 42 in order topermit the absorption length to be as short as possible. This willpermit substantially complete absorption of the pumping illuminationwithin a very short length of the ND:YAG crystal 44. As can be seen fromFIG. 10, the wavelength 0.58 microns is best suited for pumpingillumination, although the wavelengths 0.75 and 0.81 microns arerelatively well suited.

Referring now to FIG. 11A, which is an energy level diagram for theND:YAG crystal 44, it will be understood that, when pump light at theabsorption wavelength, described above, is absorbed by the ND:YAGcrystal 44, the neodymium ions are excited from the ground state to thepump band. From the pump band, the ions quickly relax, through phononinteractions, to the upper lasing level. From this upper lasing level,the neodymium ions will undergo a relatively slow fluorescence to thelower lasing level. From this latter level, a final, rapid phononrelaxation occurs to the ground state. This latter rapid relaxation in afour-level laser system of the type shown in FIG. 11A is advantageous,since the rapid phonon relaxation between the lower lasing level and theground state provides a practically empty lower lasing level. Thisfeature is shown in FIG. 11B, in which the population densities at thepump band upper lasing level, lower lasing level, and ground state areshown for the ND:YAG crystal 44 during continuous pumping. Because therate of fluorescence between the upper lasing level and lower lasinglevel is relatively slow in comparison with the phonon relaxationbetween the pump band and the upper lasing level, as well as between thelower lasing level and the ground state, the population density at theupper lasing level is substantially higher than that at the lower lasinglevel, yielding a high inversion ratio. The average lifetime ofneodymium ions at the upper lasing level, prior to spontaneousfluorescence, is about 230 microseconds at 300°k in ND:YAG.

The signal to be amplified is selected to have a wavelength at the lasertransition wavelength 1.064 microns), i.e., the wavelength of lightemitted the ND:YAG ions during relaxation between the upper and lowerlasing levels. When this signal is supplied to the crystal 44 by thecoupler 10 (FIG. 9), it will trigger the emission of stimulated photonsat the same frequency, coherent with the signal, and the signal isthereby amplified. Thus, the passage of light at this frequency willcause a photon emitting relaxation between the upper lasing level andlower lasing level of FIG. 11A, in phase with the light signal to beamplified, yielding an effective gain for the input light signal.

The gain which can be achieved in the amplifier of this invention isdependent upon the density of the inverted neodymium ion populationwithin the ND:YAG crystal 44. Initially, the ultimate inversionpopulation is limited by the lattice structure of the YAG material 44itself since in ND:YAG material, some yttrium atoms are replaced withneodymium atoms in the crystal lattice. Only approximately 1 yttriumatom in each 100 yttrium atoms may be replaced by a neodymium ionwithout distorting the lattice structure on the ND:YAG material.

Theoretical calculations of the small gain signal (g₀) of the amplifierof this invention can be made, using the relation g₀ =σΔN, where σ isthe stimulated emission cross section, for ND:YAG, 8.8×10⁻¹⁹ cm², and ΔNis the population inversion density given by: ##EQU1## where P_(p) isthe absorbed pump power, V is the crystal volume and thus, P_(p) /V isthe absorbed pump power per unit of fiber volume, t_(sp) is thespontaneous radiative lifetime, that is, the 230-microsecondfluorescence relaxation time of the neodymium ions, η₁ is the effectivespectral overlap of pump output with an ND:YAG absorption line, as shownin FIG. 10, η₂ is equal to the quantum efficiency of 1.06 -micronfluorescence, namely 0.63, and hν is equal to the energy of one pumpphoton.

Combining the above relationship provides: ##EQU2## for the dependenceof gain on pump power. It should be recognized that the value P_(p) isthe absorbed pump power and that an increase in the length of thecrystal 44 does not necessarily increase the gain. Thus, if the pumpingradiation from the source 42 is coupled completely to the ND:YAG crystal44, and travels in the crystal 44 a distance which is sufficient topermit this crystal 44 to nearly completely absorb the pumpingradiation, then the value P_(p) in this equation may be replaced by theinput power level. To obtain the net gain, however, one must subtractfrom g₀ the propagation losses within the ND:YAG crystal 44 at 1.06microns. A loss of 100 db per kilometer would reduce the gain by only0.001 db per centimeter. Thus, if the overall length of the crystal 44can be maintained relatively short, while still absorbing substantiallyall of the input pump power, the propagation losses within the amplifiercan be maintained at a low level.

DETAILED OPERATION OF THE AMPLIFIER

Referring again to FIG. 9, the pumping source 42 coupled to the fiber12A at the port A of the coupler 10, through the multiplexing action ofthe coupler 10, provides pumping illumination for the ND:YAG crystal 44.The pumping source 42 may be, for example, a long-life LED, such asthose currently available which operate at a current density ofapproximately 1,000 amps per cm² and have a radiance of approximately 5watts per sr.cm². In fact, some LEDs have been reported with a radianceof approximately 50 watts/ sr./cm². Because of the size differentialbetween the single mode fiber 12A and these LEDs, a lens may be usefulin focusing the output of the LED source into the fiber 12A.

Alternatively, the pump source 42 may be a laser diode which permitseven higher concentrations of pump power in the fiber 12A and thus inthe ND:YAG crystal 44.

Regardless of the type of pumping source 42 utilized, the efficiency ofthe system will be enhanced if the wavelength of the radiation from thissource 42 corresponds with a peak in the absorption spectrum of theND:YAG crystal 44, shown in FIG. 10. Electroluminescent diodes arecommercially available with appropriate dopings to emit spectra in the0.8 micron range which match quite well the absorption spectrum of roomtemperature ND:YAG material. For example, commercially available GaAlAsLEDs provide radiation spectra which are strong at the 0.8 micronregion. Similarly, laser diode structures are commercially availablewhich emit energy over the 0.8 to 0.85 micron range. In addition, thepump wavelength should be as close to the signal wavelength as allowedby the spectroscopy of the ND:YAG, to maximize the overall pumpingefficiency.

It will be recalled that the lasing frequency of the ND:YAG material ofthe crystal 44 is 1.06 micron. The multiplexing coupler 10 is thusfabricated for use in this invention to provide virtually completecoupling at the wavelength of the pumping source 42, 0.8 microns in theabove example, while providing substantially no coupling at the lasingfrequency of the ND:YAG crystal 44, 1.06 microns in this same example.

This selective coupling is accomplished, in accordance with thetechniques described above, by properly selecting the fiber spacing H toyield a large wavelength dependence for wavelengths between 0.8 micronsand 1.06 microns, and then by selecting a radius of curvature for thefibers 12A,12B which yields a resolution equal to the difference between1.06 and 0.8 microns, or 0.26 microns. After the resolution of thecoupler has been set in this manner, the coupler may be tuned, aspreviously described, to adjust the coupling length for the wavelengths0.8 microns and 1.06 microns so that the effective interaction length isan even multiple of the coupling length for one of this pair ofwavelengths and an odd multiple of the coupling length for the remainingwavelength. In the example shown in FIG. 9, since it is desired tocouple the output of the pump source 42 into the fiber 12B, theeffective interaction length for the coupler should be adjusted to be anodd multiple of the coupling length at the wavelength of the pump source42, i.e., 0.8 microns, and to be an even multiple of the signalfrequency 1.06 microns. This will result in a complete coupling of theillumination from the pump source 42, from the fiber 12A into the fiber12B, with essentially no coupling of the signal to be amplified from thefiber 12B to the fiber 12A. It will be understood, of course, that nocoupling in this instance means an even number of complete couplings sothat, for example, if the effective interaction length at the region 32is twice the coupling length at 1.06 microns, the signal to be amplifiedwill be coupled two complete times, once from the fiber 12B to the fiber12A, and then from the fiber 12A to the fiber 12B. If this signal entersthe coupler at port C, as shown on the left of FIG. 9, it will exituncoupled at port D. However, at port D, the signal to be amplified willcoexist with light from the pumping source 42, which will be completelycoupled from the fiber 12A to the fiber 12B.

Since light from the pumping source 42 will be transmitted along thefiber 12B, after coupling, this pumping illumination will invert theneodymium ions in the ND:YAG crystal 44. Thus, a signal, which isinjected at port C and exits, uncoupled, from port D, will be amplifiedin the manner previously described as it passes through the crystal 44,since this signal will excite spontaneous lasing relaxation of theND:YAG material of the crystal 44, which lasing relaxation will providelight coherent with the signal to be amplified.

The amplifier of the present invention therefore provides a convenientmeans to transfer pumping illumination from the pump source 42 bywavelength dependent coupling to the ND:YAG crystal 44, whileprohibiting coupling of the signal to be amplified from the fiber 12B tothe fiber 12A. It should be recognized that the results achieved in thepresent invention may also be realized using a coupler in which thecoupling efficiency at the pumping illumination wavelength is 0%, whilethe coupling efficiency at the light signal wavelength is 100%. In thiscase, the pumping source would be coupled to port C of the coupler 10,while the input light signal to be amplified would be coupled to port A.

BIDIRECTIONAL SYMMETRY

In order to make the amplifier symmetrically bidirectional, a pair ofpump sources 42,48 should be utilized, as shown in FIG. 12, along with apair of multiplexing couplers 50,52. It will be understood that, if suchbidirectional symmetry is not necessary, either of the pump sources42,48 will invert ions at one end of the ND:YAG crystal 44 and will thusyield gain for signals transmitted in either direction in the crystal44.

If only one of the pump sources 42,48 is utilized, it should berecognized that the ND:YAG crystal 44 will not be uniformly illuminated.Thus, the inverted population of neodymium ions will not be uniformlydistributed along the length of the crystal 44. Because this non-uniformor non-symmetrical state within the amplifier may yield different gainfor signals input at the fiber 54, than for signals input at the fiber56 (particularly when these signals occur simultaneously), it isadvantageous to utilize the pair of sources 42,48.

The phenomenon of dissimilar gain for signals traversing the fibercrystal 44 in different directions with a non-symmetrical inversionpopulation of neodymium ions occurs as follows. It will be recognizedthat, as a signal to be amplified propagates from left to right in thecrystal 44 of FIG. 12, it will trigger the emission of stimulatedphotons within the ND:YAG crystal 44. Such triggering emission, ofcourse, lowers the inversion population within the crystal 44. If, forexample, in a gyroscope, a pair of waves propagate simultaneouslythrough the crystal 44 in opposite directions, the signal input at theleft end will deplete the inversion population adjacent this end beforethe signal input at the right end arrives at the left end of the crystal44, as viewed in FIG. 12. If the inversion population is higher at theleft end of the crystal 44, than at the right end, as would be the caseif only the pump source 42 were utilized, the signal input at the leftwill undergo a greater amplification, since it will deplete theinversion population before the signal which is input at the right endarrives at the high density left end.

It should also be recognized that the pumping illumination supplied bythe pump sources 42,48 should be sufficient, on a continuing basis, toreplace the depleted population within the crystal 44 which occurs whenthe signals are amplified. Thus, for example, in a gyroscope where apulse signal circulates through a kilometer of fiber, acounter-propagating signal will traverse the amplifier, shown in FIG.12, approximately once each 5 microseconds. If continuous pump sources42,48 are used, they should provide sufficient output so that, duringeach 5-microsecond period, they are capable of reinverting the neodymiumion population which is relaxed during each successive traverse of thesignals to reinvert a population equal to that which has relaxed, suchthat the amplification factor or gain of the amplifier will remainrelatively constant.

As will be recognized from the above-description, a proper selection offiber spacing and radius of curvature will yield a coupler which permitspumping sources 42,48 to illuminate the crystal 44 to invert theneodymium population therein. With a proper selection of the couplerparameters, the signal to be amplified is not coupled from the fibers54,56, and thus traverses the crystal 44 to be amplified by stimulatingrelaxation of neodymium ions in the crystal 44 which produces lightcoherent with the signal to be amplified.

SIGNAL INSERTION

Referring now to FIG. 13, a system arrangement is shown in which themultiplexing properties of a pair of couplers 58,60 is used to permit apumping source 42 to illuminate the ND:YAG crystal 44 while alsopermitting an injection of signals into a recirculating fiber opticsystem. In this instance, for example, it may be assumed that the fiberends 62,64 are a part of a recirculating fiber loop and that the system,in addition to supplying pumping illumination for the crystal 44, toamplify signals circulating in this loop, must supply signals forinjection into the loop.

In the system shown in FIG. 13, the coupler 58 is arranged to couplelight wavelengths from the illumination source 42 with a couplingefficiency of 100%, while leaving the input signal at an input fiber 66effectively uncoupled. This will yield a combination signal on the fibersegment 68 which includes both the pumping illumination from the source42 and the input signal from the fiber 66. If the characteristics of thecoupler 60 are properly selected in the manner described above, and thiscoupler 60 is properly tuned, the coupler 60 may be arranged to couple100% of light wavelengths from the pumping source 42 but only 1% ofwavelengths at the signal input from the fiber segment 68. Thus, 1% ofthe input signal from the fiber segment 66 will be injected into thefiber recirculating loop and will initially be transmitted through thecrystal 44 to the fiber segment 64 for recirculation. At the same time,100% of the pumping illumination from the source 42 will injected intothe end of the crystal 44 to provide amplification for the recirculatingsignal.

When this recirculating signal appears at the fiber segment 62, only 1%of the signal will be lost at the coupler 60 and this 1% may supply anoutput for a sensor 70 to monitor the recirculating light. The remaining99% of the recirculating light signal will be injected into the crystal44 from the coupler 60, for reamplification and recirculation within thefiber loop. This system, therefore, with the use of two couplers 58,60,provides continuous sensing of the light circulating in the fiber loop,signal injection into the loop, and continuous pumping for theamplifying crystal 44. The coupling efficiencies provided above areexemplary only, but it should be recognized that the coupling efficiencyof the coupler 60 should be maintained relatively low at the wavelengthof the signal propagating in the loop so that only a small portion,smaller than the amplification added to the signal at the crystal 44, issubtracted from the recirculating signal by the coupler 60.

SUMMARY

The combination of the multiplexing coupler with an ND:YAG amplifyingcrystal permits end pumping of the amplifying crystal with simultaneoussignal injection at the end of this fiber so that the pumpingillumination need not be carefully timed with respect to the signal tobe amplified. Symmetrical bidirectional amplification is possible andsignal injection using the same multiplexing coupler may be achieved.

What is claimed is:
 1. A fiber optic amplifier system, comprising:alength of crystal fiber doped with material which will lase; a source ofsignals at a lasing frequency of said material which will lase; a sourceof light for pumping said material which will lase; an optical fibercoupled at a first end to an end of said crystal fiber; and means forcoupling both said signals and said light for pumping into said opticalfiber.
 2. A fiber optic amplifier system, as defined in claim 1, inwhich said means for coupling comprises an optical coupler.
 3. A fiberoptic amplifier system, as defined in claim 2, wherein said opticalcoupler comprises a single mode optical coupler.
 4. A fiber opticamplifier system, as defined in claim 2, in which said optical couplerutilizes evanescent field coupling.
 5. A fiber optic amplifier system,as defined in claim 2, in which said optical coupler provides differentcoupling coefficients for said signals and said light for pumping.
 6. Afiber optic amplifier system, as defined in claim 5, in which saidoptical coupler has a coupling efficiency which is wavelength dependentand in which said signals and said light for pumping are at differentwavelengths, yielding different coupling efficiencies for said signalsand said light for pumping.
 7. A fiber optic amplifier, comprising:afiber optic coupler including a pair of optical fibers juxtaposed toprovide coupling of light at a first frequency between said fibers andto prohibit coupling of light at a second frequency between said fibers;a source of pumping illumination coupled to a first end of one of saidpair of fibers, said pumping illumination being at said first frequency;a source of a signal to be amplified coupled to a first end of the otherof said pair of fibers, said signal to be amplified being at said secondfrequency; and a length of crystal fiber formed of material which willpossess a laser transition at the frequency of said signal to beamplified if said material is pumped with said pumping illumination,said crystal coupled at one end to a second end of said other of saidpair of fibers.
 8. A fiber optic amplifier, as defined in claim 7, inwhich said fiber optic coupler has an effective interaction length atthe juxtaposition of said optical fibers which is an even multiple ofthe coupling length of said fibers at said juxtaposition at thewavelength of one of said signals to be amplified and said pumpingillumination and an odd multiple of the coupling length of said fibersat said juxtaposition at the wavelength of the other of said pumpingillumination and said signals to be amplified.
 9. A fiber opticamplifier, as defined in claim 8, wherein said pair of optical fibersare laterally offset from one another to tune said coupler to thewavelength of said signals to be amplified and said pumpingillumination.
 10. A fiber optic amplifier, as defined in claim 9,wherein said pair of optical fibers are arcuate and wherein the radiusof said arcuate optical fibers is selected in accordance with thewavelength difference between said pumping illumination and said signalsto be amplified.
 11. A method of amplifying a light signal carried by anoptical fiber, comprising:combining said light signal and pumpingillumination on a single optical fiber; and coupling said combined lightsignal and pumping illumination from said single optical fiber to oneend of a crystal fiber doped with material which will emit stimulatedradiation at the frequency of said light signal if pumped with saidpumping illumination.
 12. A method of amplifying a light signal, asdefined in claim 11, wherein said combining step comprises multiplexingof said light signal and said pumping illumination in an optical couplerwhich is optically connected to said single optical fiber and which hasa coupling efficiency which is wavelength dependent.
 13. A method ofamplifying a light signal, as defined in claim 12, wherein saidmultiplexing step comprises:juxtaposing a pair of optical fibers toprovide an interaction length; and applying said light signal to one ofsaid fibers and said pumping illumination to the other of said fibers.14. A method of amplifying a light signal, as defined in claim 11,wherein the step of coupling comprises coupling said combined lightsignal and pumping illumination to a crystal fiber having a diameterwhich is less than the absorption length of said crystal fiber at thewavelength of said pumping illumination.